1. Field of the Invention
The present invention relates to a light emitting diode (LED) driving device, and more particularly, to an LED driving device suitable for high power output to drive at least one LED module.
2. Description of the Prior Art
Light emitting diodes (LED) are light emitting elements manufactured of semiconductor materials. Unlike the conventional lighting, the LED belongs to cold lighting and has plenty of advantages, such as high brightness, high lighting efficiency, uncomplicated driving circuits, low power consumption, and fast response. The LED has gradually replaced the conventional lighting.
Circuit designs of the LED driving device usually adopt a design for isolating the primary-side from the secondary-side of a transformer for safety considerations. Please refer to FIG. 1. An input voltage Vin is rectified by a bridge rectifier circuit 110 to generate a primary-side voltage V1. Herein the primary-side voltage V1 is approximately √{square root over (2)} times the input voltage Vin. For example, when the input voltage Vin is an alternating-current voltage of 110V (volt), the primary-side voltage V1 rectified by the bridge rectifier circuit 110 is a direct-current voltage of about 156V. A pulse width modulation (PWM) controller 120 controls a power switch Q1 through an output PWM signal Vg to make the primary-side voltage V1 of a transformer 130 transform into a secondary-side of the transformer 130. Therefore, an output voltage Vo is generated to light the LED serially connected to the output end. Taking flyback topology as an example, when the power switch Q1 is turned on, energy is stored in a magnetized inductance LP of the primary-side of the transformer 130. At this time, the secondary-side is turned off. When the power switch Q1 is turned off, the energy stored in the magnetized inductance LP of the primary-side of the transformer is released to the secondary-side for further generating the output voltage Vo. Herein the output voltage Vo is a direct-current (DC) voltage.
A current signal ILED flowing through the LED determines the voltage level of a compensation pin COMP of the PWM controller 120 according to a linear voltage regulator TL and a photo coupler 140. The PWM controller 120 adjusts the PWM signal Vg according to the voltage level of the compensation pin COMP, that is, to adjust the duty cycle of the power switch Q1. In other words, the current signal ILED is stabilized according to a reference voltage Vref of the linear voltage regulator TL and a resistor RLED, which means ILED=Vref/RLED. Therefore, if the number of LEDs at the output end increases, the output power becomes larger, the duty cycle of the PWM signal Vg for controlling the power switch Q1 becomes larger, and vice versa.
When being applied to high power output (i.e., there are many LEDs connected to the output end), a power factor correction circuit (PFC) 150 is usually added into the front-end to satisfy current harmonic standards, as is shown in FIG. 2.
Please refer to FIG. 2. The AC input voltage Vin is adjusted by the bridge rectifier circuit 110 and the PFC circuit 150 to generate a DC primary-side voltage V2. For example, if the input voltage Vin is 110V (AC), the generated primary-side voltage V2 is about 200V (DC). If the input voltage Vin is about 220V (AC), the generated primary-side voltage V2 is about 400V (DC).
There are two feedback paths inside the PFC circuit 150. One path is a current feedback path 152 used for making the waveform of the input current Iin follow the waveform of the input voltage Vin and be in phase with the input voltage Vin to improve the power factor to satisfy current harmonic standards. The other path is a voltage feedback path 154, which adjusts the input current Iin through the feedback of the primary-side voltage V2 to further stabilize the primary-side voltage V2.
However, the circuit design for such driving circuit must utilize the linear voltage regulator and the photo coupler to isolate the primary-side from the secondary-side. In addition, in high power output (greater than 150 W) applications, a transformer of large-scale is necessary, which results in increasing cost, occupying space, and accompanying complicated thermal dissipation problems. Besides, the whole driving circuit must use two independent controller ICs (integrated circuit), i.e., the PFC controller and the PWM controller. Not only circuit designs become more complicated, but also cost becomes more expensive.